The molecular basis of D1 degradation and photosystem two repair

Plant growth is exquisitely sensitive to the intensity of light in the environment. At low light intensities, growth is limited because of insufficient light to drive photosynthesis. At higher intensities, growth becomes inhibited because of the damaging effects of light on the photosynthetic apparatus found in the chloroplast. This phenomenon of photoinhibition has a profound effect on crop yields especially when plants are subjected to high light in combination with other stress conditions such as heat and water stress. One of the targets of light damage in the chloroplast is the photosystem two complex, which is involved in water oxidation and photosynthetic electron transport, and in particular the D1 protein. Plants have developed a repair mechanism to replace damaged D1 by a newly synthesised copy. Under most conditions this repair mechanism allows PSII activity to be maintained. However at high light intensities the rate of repair is unable to match the rate of damage to PSII and under these conditions there is a net loss of PSII activity and photosynthetic performance. In principle, improving the repair mechanism is one route by which more light-resistant plants can be generated. As yet the molecular details of PSII repair are unclear, especially the process by which damaged D1 is removed from the membrane. Recently we discovered the identity of a particular class of protease (FtsH proteases) that was needed for D1 degradation. We have also discovered that the N-terminus of D1 is important for D1 degradation in cyanobacteria, which are closely related to chloroplasts. This has led us to postulate a mechanism for D1 degradation that involves the recognition of the N-terminus of damaged D1 by the FtsH protease. Our hypothesis is at odds with the current model in the scientific literature. In this proposal we have designed a series of experiments to differentiate between the two models. Ultimately the results of this work will provide a clearer picture of PSII repair in plants.

We have recently proposed an 'FtsH-only' model for the degradation of damaged D1 protein during repair of the photosystem two complex in vivo. The 'FtsH-only' model, which is now supported by a variety of published and unpublished data, predicts that hetero-oligomeric FtsH complexes degrade full-length damaged D1 in a highly processive reaction initiated from the N-terminus. In contrast, the earlier DegP2/FtsH model predicts that D1 is first cleaved by the DegP2 protease into 23-kDa N-terminal and 10-kDa C-terminal fragments, which are then degraded by FtsH. The overall aim of this proposal is to test these two models and to investigate the role of the N-terminal region of D1 and N-terminal D1 phosphorylation in D1 degradation in chloroplasts. Objectives are to: (1) test the physiological significance of DegP2 for PSII repair in vivo using T-DNA-tagged lines of Arabidopsis thaliana in which DegP2 is non-functional (2) test the importance of the N-terminal region of D1 for D1 degradation in site-directed mutants of Synechocystis 6803 and tobacco (3) test the physiological importance of D1-Thr2 phosphorylation in chloroplasts (4) establish an in vitro system for assaying FtsH-mediated degradation of damaged D1.